Box Ediacaran arthropods

Are they or aren't they? Some paleontologists believe they can identify some of the Ediacaran animals as arthropods or proto-arthropods; others dispute this. Parvancorina (Fig. 14.2), for example, is a possible candidate, with its shield-shaped outline, strong axial ridge and arched anterior lobes, together with a convex profile. It really looks like a juvenile trilobite molt stage, but did not have a mineralized skeleton. Not convinced? Beautifully preserved fossils from a new Cambrian Lagerstätte, the Chinese Kaili fauna seem to confirm it. Specimens of the genus Skania, first described from the Burgess Shale, have many similarities to Parvancorina but this genus has an exoskeleton and a better-defined cephalon and dorsal trunk (Lin et al. 2006). Skania together with Parvancorina and Primi-caris may have formed a sister group to the Arachnomorpha (which includes the spiders). Moreover this relationship establishes a Proterozoic root for the Arthropoda and is the first arthropod crown group that demonstrably ranges through the Precambrian-Cambrian boundary. A Proterozoic origin for the arthropods may help pin down more precisely their time of divergence from the last common ancestor of the arthropods and priapulid worms.

With the exception of the agnostids and eodiscids, which have two and two to three thoracic segments, respectively, trilobites are polymeric, that is usually having up to about 40 thoracic segments. The trilobite pygidium is usually a plate of between one and 30 fused segments. Most Cambrian trilobites have small, micropygous pygidia, whereas later forms are either heteropygous, where the pygidium is smaller than the cephalon, eye anterior border glabellar furrow / _ glabella free cheek facial occipital-ring genal_

Figure 14.3 Trilobite morphology: (a) external morphology of the Ordovician trilobite Hemiarges; (b) generalized view of the anterior of the Silurian trilobite Calymene revealing details of the underside of the exoskeleton; and (c) details of the limb pair associated with a segment of the exoskeleton.

gill-bearing limb walking limb

Figure 14.3 Trilobite morphology: (a) external morphology of the Ordovician trilobite Hemiarges; (b) generalized view of the anterior of the Silurian trilobite Calymene revealing details of the underside of the exoskeleton; and (c) details of the limb pair associated with a segment of the exoskeleton.

to become the glabella. The next, meraspis, stage has a discrete, transitory pygidium where thoracic segments form at its anterior margin and are released at successive molts to form the thorax. The holaspis stage has a full complement of thoracic segments for the species but growth continues through further molts and maturity may not be reached until some time after the holaspis stage was reached. Clearly in many trilobite-dominated faunas, counts of skeletal remains will significantly over-represent the relative numbers of living animals in the community. Many researchers divide the number of exuviae by about six to eight to obtain a more realistic census of the trilobite population in a typical community (see p. 93).

During times of stress, to avoid unpleasant environmental conditions or perhaps an attentive predator, most trilobites could roll up like a carpet. During the Paleozoic, a number of groups, including asaphids, calymenids, pha-copids and trinucleids (see p. 374), evolved a variety of sophisticated structures to enhance this behavior, although Cambrian taxa probably had a limited ability to curl up. Spheroidal enrolment involved articulation of all the thoracic segments to form a ball, whereas in the less common discoidal mode of enrolment the thorax and pygidium were merely folded over the cephalon. Cambrian trilobites could certainly enrol, but it was not until the Ordovician that true coaptative structures, locking parts of the skeleton against each other, first appeared. For example, in the phacopids, tooth and socket pairs were developed on the cephalic and pygidial doublure, respectively; these opposing structures clicked together to hold the trilobite in a tight ball, presenting only the exoskeleton to the world outside (Bruton & Haas 2003).

or macropygous, where the pygidium is larger.

Like virtually all arthropods the trilobites grew by ecdysis or molting (Fig. 14.6). Ontogeny involved the periodic discarding of spent exoskeletons or exuviae. Initial molt stages were quite different from those of adults. After a phaselus larval stage that swam freely in the plankton, the protaspis stage is a minute disk with a segmented median lobe destined

Main trilobite groups and lifestyles_

Although some workers have split the trilobites into two orders, the Agnostida and the Polymerida, most currently recognize about nine orders of trilobite based on a spectrum of characters, including the anatomy of their ontogenetic stages and more recently the location and morphology of the hypostome. In the most primitive conterminant condition, the hypostome is similar in shape to the glabella and is attached to the anterior part of the doublure. Natant hypostomes were not attached to the skeleton, whereas the impendent hypostome was attached to the doublure, but its shape was quite different from the glabella above.

Some authorities have excluded the distinctive agnostids from the Trilobitomorpha and there is now strong evidence to suggest they were crustaceans. Agnostids were small to minute, usually blind animals with subequal cephala and pygidia and only two thoracic segments; they were probably planktonic, which may account for their very wide distribution.

The redlichiids include Olenellus, with 1844 spiny thoracic segments and typical of the Atlantic province, Redlichia itself, more typical of the Pacific province, and the large, spiny, micropygous Paradoxides, common in the high latitudes of the Mid Cambrian.

Corynexochid trilobites were a mixed bag of taxa; the order includes genera with con-terminant hypostomes such as Olenoides and large smooth forms such as Bumastus and Illaenus, having impendent hypostomes.

The lichids contain mainly spiny forms with conterminant hypostomes. Apart from Lichas itself the order also includes the spiny odontopleurids such as Leonaspis.

Phacopids were mainly proparian trilobites with schizochroal eyes (Box 14.3) and lacking rostral plates that ranged from the Lower Ordovician to Upper Devonian. The order includes the large tuberculate Cheirurus, Calymene with a marked gonatoparian suture, and Dalmanites with long genal spines, kidney-shaped eyes, spinose thoracic segments and the pygidium extended as a long spine.

The ptychopariids are all characterized by natant hypostomes and include some specialized groups. For example Triarthrus was modified for burrowing, Conocoryphe was blind and Harpes had a sensory fringe round the cephalon.

Asaphids had either conterminant or impendent hypostomes and include Asaphus

Box 14.3 Vision in trilobites: from corrective lenses to sunshades

Trilobites have the oldest known visual system based on eyes: paleontologists can even look through the ancient lenses and see the world as trilobites saw it! Trilobite eyes are compound, consisting of many lenses, just like those of the crustaceans and insects. Euan Clarkson's classic studies (1979) emphasized the functions of the two main types of lens arrangement found in trilobites (Fig. 14.4). The trilobite eye generally consists of many lenses of calcite with the c-axis (the main optical axis) perpendicular to the surface of the eye. The more primitive and widespread holochroal eye has many close-packed lenses, all about the same size, covered by a single membrane. The more advanced and complex schizochroal condition has no modern analog and has larger, discrete lenses arranged in rows or files. It is uncertain how this system operated in detail; presumably it offered higher-quality images than those of the holochroal systems. Moreover both mature holochroal and schizochroal configurations apparently developed from immature schizochroal conditions. Thus early growth stages of holochroal eyes in quite different groups such as those of the Cambrian eodiscid Shizhu-discus with the oldest visual system in the world and Phacops from the Devonian with a schizochroal system have broadly similar arrangements, suggesting that the latter system developed by pedomor-phosis (see p. 145). A third, less well known optical system, abathochroal, is confined to a short-lived Cambrian group, the eodiscids (most of which were blind). Less is known about them than other visual systems and their origins remain obscure.

But could such visual systems cope with bright sunlight? Probably not, and this suggests that many groups were nocturnal. But not all. A remarkable Devonian phacopid trilobite, Erbenochile, from Morocco, actually has a type of sunshade covering the top of a column of lenses (Fortey & Chatterton 2003). The animal could scan the seafloor for potential prey without the distraction of direct sunlight.

Figure 14.5 Facial sutures: the tracks of the proparian, gonatoparian and opisthoparian sutures. The lateral suture (not illustrated) follows the lateral margin of the cephalon.

proparian gonatoparian opisthoparian

Figure 14.5 Facial sutures: the tracks of the proparian, gonatoparian and opisthoparian sutures. The lateral suture (not illustrated) follows the lateral margin of the cephalon.

and Ceratopyge together with pelagic forms such as Cyclopyge and Remopleurides, and the stratigraphically important trinucleids such as Onnia, Cryptolithus and Tretaspis.

The proetids were isopygous forms with large glabellae and long hypostomes having genal spines and large holochroal eyes. The group ranged from the Lower Ordovician to the Upper Permian. Proetus was a small form with a relatively large, inflated and often granular glabella, known from the Ordovician to the Devonian. Phillipsia, one of the youngest members of the order, was a small isopygous genus with large crescent-shaped eyes and an opisthoparian suture.

The naraoids, including Naraoia itself and Tegopelte, have often been included with the trilobites. They were not calcified and lacked thoracic segments. The group was restricted to the Middle Cambrian. Naraoia was first described from the Burgess Shale as a bran-chiopod crustacean, but it has only more

recently been reclassified as a soft-bodied trilobite. It is now known from other Cambrian Lagerstätten together with a number of related taxa. The group is probably a sister group to the trilobites + agnostids (Edgecombe & Ramsköld 1999) and recent cladistic analyses confirm this phylogenetic position, basal to Trilobitomorpha, and within the larger clade Arachnomorpha (Cotton & Braddy 2004).

Trilobite morphology is hugely variable, presumably reflecting their broad range of adaptations (Fig. 14.7). Most trilobites were almost certainly benthic or nektobenthic, leaving a variety of tracks and trails in the marine sediments of the Paleozoic seas (see Chapter 19). With the exception of the pha-copids that may have hunted, the simple mouthparts of the trilobites suggest a diet of microscopic organisms and a detritus-feeding strategy.

Many trilobites developed spinose exoskel-etons. The spines reduce their weight : area ratio and this suggested that these trilobites adopted a floating, planktonic life strategy, supposedly backed up by the fact they occasionally had inflated glabellae. More recently, however, the suggestion that their glabella was filled with gas has been shown to be a little fanciful, and it seems more likely that these forms used their long spines to spread the weight on a soft muddy substrate. Downward-directed spines probably held the thorax and pygidium well above the sediment-water interface. In some forms, the spines probably aided shallow burrowing when the body flexed. Spines are most extravagantly developed in the odontopleurids.

Some trilobites such as Cybeloides and Encrinurus evolved eyes on stalks or others, for example Trinucleus, lost them altogether in favor of possible sensory setae (stiff hairlike structures). These specialized forms may have periodically concealed themselves in the sediment. Trimerus had a cephalon and pygid-ium fashioned in the shape of a shovel that might have helped it plow through the sediment. The cyclopygid Opipeuter, from the Lower Ordovician of Spitzbergen, Ireland and Utah, on the other hand, seems to have been an active pelagic swimmer; it had a long, slender body with a flexible exoskeleton and large eyes, just like a modern shrimp-like amphipod, together with a widespread distribution.

Trilobites show extensive convergence: the same broad morphotypes appear repeatedly in different lineages, presumably reflecting repeats of the same life strategies. Richard Fortey and Robert Owens documented seven ecomorphic groups ranging from the turber-culate, mobile phacomorphs to the smooth, infaunal illaenimorphs (Fig. 14.8) and these were related to their wide variety of lifestyles (Fig. 14.9).

Distribution and evolution: trilobites in space and time_

Trilobite faunas have formed the basis for many paleogeographic reconstructions of the Cambrian and Ordovician world. During the Cambrian, biogeographic patterns were complex, but some provinces have been defined, such as the high-latitude Atlantic region (with redlichiids) and the low-latitude Pacific region (with olenellids). Statistical analysis of Ordovician trilobite faunas in the early 1970s established a low-latitude bathy-urid province (Laurentia), an intermediate to high-latitude asaphid province (Baltica) and a

high-latitude Selenopeltis province (Gond-wana). Despite a number of modifications, this basic pattern is generally accepted (see also Chapter 2).

Some Early Paleozoic trilobite communities may also be interpreted as showing an onshore-offshore spectrum, from shallow-water illaenid-cheirurid associations to deep-water olenid communities (Fig. 14.10). In general terms, the shallow-water, pure car bonate, illaenid-cheirurid communities apparently lasted the longest.

Trilobites (such as Choubertella and Schmidtiellus) first appeared in the Early Cambrian and the group survived until the end of the Permian, when the last genera, such as Pseudophillipsia, disappeared (Fig. 14.11). In a history of 350 million years, the basic body plan was essentially unchanged, but many modifications promoted trilobite

Figure 14.9 Lifestyles of the trilobites: a mosaic of selected Lower Paleozoic trilobites in various life attitudes.

abundance and diversity. Not surprisingly the Trilobitomorpha has been a major source of evolutionary data and there have been many studies on the functional morphology of the group (e.g. Bruton & Haas 2003).

Trilobites have provided key evidence in studies of macroevolution, especially in the controversy over punctuated equilibrium and phyletic gradualism (see Chapter 5). Trilo-bites have complex morphologies that can be easily measured and analyzed statistically (Box 14.4). The studies of Niles Eldredge and Stephen Jay Gould on the number of lens files of the Devonian trilobite Phacops rana formed the basis for their punctuated equilibrium model. On the other hand Peter Sheldon's investigation of over 15,000 specimens of Mid Ordovician trilobites demonstrated gradual changes in the number of pygidial ribs, possibly a slower, adaptive, fine-tuning to more stable environments. Euan Clarkson's survey of microevolutionary change in Upper Cambrian olenid trilobites from the Alum Shales of Sweden provided evidence of similar gradual change (Fig. 14.13). Macroevolution-ary change in trilobites was effected by heter-ochrony (see p. 145). Pedomorphosis during ontogeny of the animal as a whole or applied to particular organs such as the eyes generated new species and new biological structures.

Trilobites show a number of evolutionary trends. Through time, for example, those tri-lobites that adopted enrolment as a defensive strategy became better at it: the spines and sockets around their exoskeletons came to fit and lock better and better. Early trilobites probably rolled up into a rough ball, but could be prized apart by a persistent predator; later enrolling trilobites were impenetrable. There was a reduction in the size of the rostral plate and in some groups there was an increase in spinosity and a trend from micropygy to isopygy. The evolution of schizo-chroal visual systems appeared, by pedomor-phosis, during the Early Ordovician in the phacopids.

Trilobite abnormalities and injuries_

Trilobites have left a rich record of abnormalities and injuries, some evidence that they faced problems during ecdysis and that they were attacked by predators (Fig. 14.14). There are three main types of abnormality (Owen 1985):

Landmarks, as the name suggests, are recognizable geographic features. Such features can also be defined on fossil organisms and they form the basis for geometric morphometries. The aim of these statistical techniques is to define precisely how shapes differ from each other, and the landmarks are the fixed points of comparison. Each landmark can be recorded as a set of coordinates or the distances between points, and they can be recorded from digital photographs or image analysis systems and stored in spreadsheets. For example, 22 landmarks were necessary to define shape variations in the exoskeletons of well-preserved Aulacopleura from the Silurian rocks of Bohemia (Fig. 14.12). The data can be used in a variety of ways. For example it is relatively easy to see, visually, how the trilobite actually grew; the most substantial growth took place in the thoracic region during ontogeny. In some studies it is necessary to translate this into quantitative terms, and landmark analysis is the key.